Ceramic raw material powder, multilayer ceramic capacitor and manufacturing method of multilayer ceramic capacitor

ABSTRACT

Ceramic raw material powder includes: a main phase having a perovskite structure, wherein elements acting as a donor and an acceptor are solid-solved in B sites of the perovskite structure, wherein a relationship of (concentration of the element acting as a donor)×(valence of the element acting as a donor−4)&lt;(concentration of the element acting as an acceptor)×(4−valence of the element acting as an acceptor) is satisfied, in a center region of each grain of the ceramic raw material powder, wherein a relationship of (concentration of the element acting as a donor)×(valence of the element acting as a donor−4)&gt;(concentration of the element acting as an acceptor)×(4−valence of the element acting as an acceptor) is satisfied, in a circumference region of each grain of the ceramic raw material powder.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2018-232703, filed on Dec. 12,2018, the entire contents of which are incorporated herein by reference.

FIELD

A certain aspect of the present invention relates to ceramic rawmaterial powder, a multilayer ceramic capacitor and a manufacturingmethod of a multilayer ceramic capacitor.

BACKGROUND

A multilayer ceramic capacitor has a structure in which each ofdielectric layers and each of internal electrode layers are alternatelystacked. The dielectric layers are formed by sintering ceramic rawmaterial powder. It is therefore preferable that the ceramic rawmaterial powder has predetermined characteristic, from a viewpoint offavorable performance of the multilayer ceramic capacitor. For example,there is disclosed a technology in which a predetermined element issolid-solved in the ceramic raw material powder in advance, in order toachieve sufficient reliability even if thicknesses of the dielectriclayers are reduced (for example, see Japanese Patent ApplicationPublication No. 2017-108128 hereinafter referred to as Document 1 andJapanese Patent Application Publication No. 2017-228737 hereinafterreferred to as Document 2).

SUMMARY OF THE INVENTION

In Document 1, barium titanate in which a donor element is solid-solvedis used. With the technology, although life of the multilayer ceramiccapacitor is improved because of donor effect, degradation of insulationcharacteristic may be a problem. In Document 2, barium titanate in whicha donor element and an acceptor element are solid-solved is used.However, with the technology, although the degradation of the insulationcharacteristic can be suppressed, the donor effect may be degraded.

The present invention has a purpose of providing ceramic raw materialpowder, a multilayer ceramic capacitor and a manufacturing method of amultilayer ceramic capacitor that are capable of achieving donor effectand acceptor effect.

According to an aspect of the present invention, there is providedceramic raw material powder including: a main phase having a perovskitestructure, wherein an element acting as a donor and an element acting asan acceptor are solid-solved in B sites of the perovskite structure,wherein a relationship of (concentration of the element acting as adonor)×(valence of the element acting as a donor−4)<(concentration ofthe element acting as an acceptor)×(4−valence of the element acting asan acceptor) is satisfied, in a center region of each grain of theceramic raw material powder, wherein a relationship of (concentration ofthe element acting as a donor)×(valence of the element acting as adonor−4)>(concentration of the element acting as an acceptor)×(4−valenceof the element acting as an acceptor) is satisfied, in a circumferenceregion of each grain of the ceramic raw material powder.

According to another aspect of the present invention, there is provideda manufacturing method of a multilayer ceramic capacitor including:forming green sheets including ceramic raw material powder of which amain phase has a perovskite structure; forming a multilayer structure byalternately stacking each of the green sheets and each of paste forinternal electrode; firing the multilayer structure, wherein an elementacting as a donor and an element acting as an acceptor are solid-solvedin B sites of the perovskite structure, wherein a relationship of(concentration of the element acting as a donor)×(valence of the elementacting as a donor−4)<(concentration of the element acting as anacceptor)×(4−valence of the element acting as an acceptor) is satisfied,in a center region of each grain of the ceramic raw material powder,wherein a relationship of (concentration of the element acting as adonor)×(valence of the element acting as a donor−4)>(concentration ofthe element acting as an acceptor)×(4−valence of the element acting asan acceptor) is satisfied, in a circumference region of each grain ofthe ceramic raw material powder.

According to another aspect of the present invention, there is provideda multilayer ceramic capacitor including: a plurality of dielectriclayers; and a plurality of internal electrode layers, wherein each ofthe plurality of dielectric layers and each of the plurality of internalelectrode layers are alternately stacked, wherein the plurality ofdielectric layers are formed by firing the above-mentioned ceramic rawmaterial powder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a perspective view of a multilayer ceramic capacitorin which a cross section of a part of the multilayer ceramic capacitoris illustrated;

FIG. 2A and FIG. 2B illustrate concentration gradients of an elementacting as a donor and an element acting as an acceptor; and

FIG. 3 illustrates a manufacturing method of a multilayer ceramiccapacitor.

DETAILED DESCRIPTION

A description will be given of an embodiment with reference to theaccompanying drawings.

Embodiment

FIG. 1 illustrates a perspective view of a multilayer ceramic capacitor100 in accordance with an embodiment, in which a cross section of a partof the multilayer ceramic capacitor 100 is illustrated. As illustratedin FIG. 1, the multilayer ceramic capacitor 100 includes a multilayerchip 10 having a rectangular parallelepiped shape, and a pair ofexternal electrodes 20 a and 20 b that are respectively provided at twoend faces of the multilayer chip 10 facing each other. In four facesother than the two end faces of the multilayer chip 10, two faces otherthan an upper face and a lower face of the multilayer chip 10 in astacking direction are referred to as side faces. The externalelectrodes 20 a and 20 b extend to the upper face, the lower face andthe two side faces of the multilayer chip 10. However, the externalelectrodes 20 a and 20 b are spaced from each other.

The multilayer chip 10 has a structure designed to have dielectriclayers 11 and internal electrode layers 12 alternately stacked. Thedielectric layer 11 includes ceramic material acting as a dielectricmaterial. The internal electrode layers 12 include a base metalmaterial. End edges of the internal electrode layers 12 are alternatelyexposed to a first end face of the multilayer chip 10 and a second endface of the multilayer chip 10 that is different from the first endface. In the embodiment, the first end face faces with the second endface. The external electrode 20 a is provided on the first end face. Theexternal electrode 20 b is provided on the second end face. Thus, theinternal electrode layers 12 are alternately conducted to the externalelectrode 20 a and the external electrode 20 b. Thus, the multilayerceramic capacitor 100 has a structure in which a plurality of dielectriclayers 11 are stacked and each two of the dielectric layers 11 sandwichthe internal electrode layer 12. In the multilayer chip 10, the internalelectrode layer 12 is positioned at an outermost layer. The upper faceand the lower face of the multilayer chip 10 that are the internalelectrode layers 12 are covered by cover layers 13. A main component ofthe cover layer 13 is a ceramic material. For example, a main componentof the cover layer 13 is the same as that of the dielectric layer 11.

For example, the multilayer ceramic capacitor 100 may have a length of0.25 mm, a width of 0.125 mm and a height of 0.125 mm. The multilayerceramic capacitor 100 may have a length of 0.4 mm, a width of 0.2 mm anda height of 0.2 mm. The multilayer ceramic capacitor 100 may have alength of 0.6 mm, a width of 0.3 mm and a height of 0.3 mm. Themultilayer ceramic capacitor 100 may have a length of 1.0 mm, a width of0.5 mm and a height of 0.5 mm. The multilayer ceramic capacitor 100 mayhave a length of 3.2 mm, a width of 1.6 mm and a height of 1.6 mm. Themultilayer ceramic capacitor 100 may have a length of 4.5 mm, a width of3.2 mm and a height of 2.5 mm. However, the size of the multilayerceramic capacitor 100 is not limited.

A main component of the internal electrode layers 12 is a base metalsuch as nickel (Ni), copper (Cu), tin (Sn) or the like. The internalelectrode layers 12 may be made of a noble metal such as platinum (Pt),palladium (Pd), silver (Ag), gold (Au) or alloy thereof.

The dielectric layers 11 are mainly composed of a ceramic material thatis expressed by a general formula ABO₃ and has a perovskite structure.The perovskite structure includes ABO_(3-α), having anoff-stoichiometric composition. For example, the ceramic material issuch as BaTiO₃ (barium titanate), CaZrO₃ (calcium zirconate), CaTiO₃(calcium titanate), SrTiO₃ (strontium titanate),Ba_(1-x-y)Ca_(x)Sr_(y)Ti_(1-z)Zr_(z)O₃ (0≤x≤1, 0≤y≤1, 0≤z≤1) having aperovskite structure. For example, the dielectric layers 11 are formedby firing ceramic raw material powder of which a main component is aceramic material having a perovskite structure.

From a viewpoint of downsizing the multilayer ceramic capacitor 100 andenlarging capacity of the multilayer ceramic capacitor 100, it isrequested that thicknesses of the dielectric layers are reduced.However, when the thicknesses of the dielectric layers 11 are reduced,life characteristic of the multilayer ceramic capacitor 100 may bedegraded and reliability of the multilayer ceramic capacitor may bedegraded.

Next, a description will be given of degradation of the reliability. Thedielectric layer 11 is formed by firing ceramic raw material powder ofwhich a main phase has a perovskite structure expressed by a generalformula ABO₃ and sintering the ceramic raw material powder. Therefore,when the ceramic raw material powder is exposed to reductive atmosphereduring the firing, oxygen defect occurs in ABO₃ of the ceramic rawmaterial powder. During operation of the multilayer ceramic capacitor100, a voltage is repeatedly applied to the dielectric layer 11. In thiscase, the oxygen defect moves, and barrier may be broken. That is, theoxygen defect in the perovskite structure is one of reasons of thereliability degradation of the dielectric layer 11.

And so, it is preferable that an element acting as a donor is included(solid-solved) in a B site of the perovskite structure. For example, theelement acting as a donor is such as Mo (molybdenum), Nb (niobium), Ta(tantalum), W (tungsten) or the like. When the element acting as a donoris solid-solved in the B site, the oxygen defect in the perovskitestructure is suppressed. Therefore, life of the dielectric layer 11 iselongated, and the reliability of the dielectric layer 11 is improved.

However, when the element acting as a donor is solid-solved in the Bsite, defect such as degradation of insulation characteristic may occur.And so, it is preferable that the element acting as a donor is solidsolved in the B site and an element acting as an acceptor is alsosolid-solved in the B site. For example, the element acting as anacceptor is such as Mn (manganese), Ni, Cu, Fe (iron), Cr (chromium), Co(cobalt), Zn (zinc), Y (yttrium), Dy (dysprosium), Ho (holmium), Er(erbium) or the like. When the element acting as an acceptor issolid-solved in the B site, a leak current is suppressed. Therefore,degradation of the insulation characteristic of the dielectric layer 11is suppressed.

However, in this case, although the degradation of the insulationcharacteristic is suppressed, donor effect is degraded. And so, in theembodiment, a description will be given of a manufacturing method of themultilayer ceramic capacitor 100, by using ceramic raw material powderwhich is capable of maintaining the donor effect and the acceptoreffect.

In the embodiment, in B sites of ceramic raw material powder having theperovskite structure, the element acting as a donor and the elementacting as an acceptor are solid-solved. And, concentration gradients ofthe element acting as a donor and the element acting as an acceptor arecontrolled.

In the embodiment, a relationship in a circumference region of theceramic raw material powder and a relationship in a center region of theceramic raw material powder are regulated, with respect to theconcentration of the element acting as a donor and the concentration ofthe element acting as an acceptor. In concrete, the concentration of theelement acting as a donor is larger than the concentration of theelement acting as an acceptor, in the circumference region. And, theconcentration of the element acting as an acceptor is larger than theconcentration of the element acting as a donor, in the center region.

In addition to the concentration of the elements solid-solved in the Bsite, a relationship between a valence (+4) of the B site of theperovskite structure of parent crystal and a valence of the elementsolid-solved in the B site influences on the donor effect and theacceptor effect. For example, with respect to the element acting as adonor, the donor effect of an element of which a valence is +6 (forexample, Mo) is larger than that of an element of which a valence is +5(for example, V (vanadium)), when the concentration of the element ofwhich the valence is +6 is the same as that of the element of which thevalence is +5. In the same manner, with respect to the element acting asan acceptor, the acceptor effect of an element of which a valence is +2(for example, Mn) is larger than that of an element of which a valenceis +3 (for example, Fe), when the concentration of the element of whichthe valence is +2 is the same as that of the element of which thevalence is +3. Therefore, it is preferable that a relative valence withrespect to the valence of the B site (+4) is considered, from aviewpoint of achieving that the concentration of the element acting as adonor is larger than that of the element acting as an acceptor in thecircumference region of the ceramic raw material powder and theconcentration of the element acting as an acceptor is larger than thatof the element acting as a donor in the center region.

And so, in the embodiment, an amount of the element acting as a donorwhich is solid-solved in the B site and an amount of the element actingas an acceptor which is solid-solved in the B site are regulated so thatthe following formula (1) and the following formula (2) are satisfied,with respect to the ceramic raw material powder.

In the center region: (concentration of the element acting as adonor)×(valence of the element acting as a donor−4)<(concentration ofthe element acting as an acceptor)×(4−the valence of the element actingas an acceptor)  (1)

In the circumference region: (concentration of the element acting as adonor)×(valence of the element acting as a donor−4)>(concentration ofthe element acting as an acceptor)×(4−valence of the element acting asan acceptor)  (2)

It is possible to, in the ceramic raw material powder, sufficientlyenlarge the donor effect of the circumference region on whichreduction/oxidation caused by the firing atmosphere/thermal treatmentatmosphere has a large influence and to reduce the amount of the oxygendefect, when the multilayer ceramic capacitor is manufactured by usingthe ceramic raw material powder satisfying the relationships. And it ispossible to sufficiently enlarge the acceptor effect in the centerregion of the ceramic raw material powder. And the insulationcharacteristic is improved. It is therefore possible to achieve both ofthe donor effect and the acceptor effect. Accordingly, it is possible tomanufacture the multilayer ceramic capacitor in which a balance betweenlife and insulation characteristic is good and reliability is excellent.

A value A is (concentration of the element acting as a donor)×(valenceof the element acting as a donor−4). A value B is (concentration of theelement acting as an acceptor)×(4−valence of the element acting as anacceptor). For example, as illustrated in FIG. 2A, in the B site of theperovskite structure, the ceramic raw material powder has a gradient inwhich the value A gradually or stepwise decreases from the circumferenceregion to the center region and the value B has an even distributionwithout a concentration gradient. For example, the concentration of theelement acting as a donor in the circumference region is larger than theconcentration of the element acting as a donor in the center region. InFIG. 2A, a dotted line schematically indicates a grain of the ceramicraw material powder. A horizontal axis indicates a position of thegrain. A first vertical axis (left axis) indicates the value A. A secondvertical axis (right axis) indicates the value B.

The ceramic raw material powder has the relationships of the formula (1)and the formula (2), in the center region and the circumference region.Therefore, as illustrated in FIG. 2B, the decreasing gradient of thevalue A may have a local maximum value, on the way from thecircumference region to the center region.

The relationships of the formula (1) and the formula (2) may be appliedto a case where two different elements acting as a donor are added tothe B sites. And, the relationships of the formula (1) and the formula(2) may be applied to a case where two different elements acting as anacceptor are added to the B sites. In these cases, a weight average ofthe two different elements may be used as the concentration of theelements acting as a donor or the concentration of the elements actingas an acceptor.

The center region of grains may be defined as a geometric gravity centerregion.

A description will be given of a reason that the element acting as adonor and the element acting as an acceptor are solid-solved in theceramic raw material powder. For example, it is thought that therelationships of the formula (1) and the formula (2) are achieved, bymixing the ceramic raw material powder, the oxide of the element actingas a donor and the oxide of the element acting as an acceptor, anddiffusing each element in a firing process. However, even if theconcentration of the element acting as a donor in the circumferenceregion of each grain can be enlarged, it is difficult to diffuse eachelement into the center region of each grain. Therefore, the elementacting as a donor and the element acting as an acceptor are solid-solvedin the ceramic raw material powder.

Next, a description will be given of a manufacturing method of themultilayer ceramic capacitor 100. FIG. 3 illustrates a manufacturingmethod of the multilayer ceramic capacitor 100.

(Making Process of Raw Material Powder)

Ceramic raw material powder, in which the main phase has a perovskitestructure expressed by a general formula ABO₃ and the element acting asa donor and the element acting as an acceptor are solid-solved in Bsites so that the formula (1) and the formula (2) are satisfied, isprepared. Various methods can be used as a synthesizing method of theceramic raw material powder. For example, a solid-phase method, asol-gel method, a hydrothermal method or the like can be used. Forexample, in a case where Mo and Mn are solid-solved in the Ti site ofBaTiO₃, BaTiO₃ in which Mn is solid-solved is made from mixed powder ofBaCO₃, TiO₂ and MnCO₃. After that, Mo compound is added to the BaTiO₃ inwhich Mn is solid-solved. And grain growth of the BaTiO₃ is performed.For example, it is preferable that an average grain diameter of theceramic raw material powder is 50 nm to 150 nm, from a view point ofthickness reduction of the dielectric layers 11.

Additive compound may be added to the resulting ceramic raw materialpowder in accordance with purposes. The additive compound may be anoxide of a rare earth element (Y (yttrium), Sm (samarium), Eu(europium), Gd (gadolinium), Tb (terbium), Dy (dysprosium), Ho(holmium), Er (erbium), Tm (thulium) and Yb (ytterbium)), or an oxide ofCo (cobalt), Ni, Li (lithium), B (boron), Na (sodium), K (potassium) andSi (silicon), or glass.

For example, the resulting ceramic raw material powder is wet-blendedwith additives and is dried and crushed. Thus, a ceramic material isobtained. For example, the grain diameter may be adjusted by crushingthe resulting ceramic material as needed. Alternatively, the graindiameter of the resulting ceramic power may be adjusted by combining thecrushing and classifying. With the processes, the ceramic materialacting as a main component of the dielectric layers are obtained.

(Stacking Process)

Next, a binder such as polyvinyl butyral (PVB) resin, an organic solventsuch as ethanol or toluene, and a plasticizer are added to the resultingdielectric material and wet-blended. With use of the resulting slurry, astripe-shaped dielectric green sheet with a thickness of 3 μm to 10 μmis coated on a base material by, for example, a die coater method or adoctor blade method, and then dried.

Then, a pattern of the internal electrode layer 12 is provided on thesurface of the dielectric green sheet by printing metal conductive pastefor forming an internal electrode with use of screen printing or gravureprinting. The conductive paste includes an organic binder. A pluralityof patterns are alternatively exposed to the pair of externalelectrodes. The metal conductive paste includes ceramic particles as aco-material. A main component of the ceramic particles is not limited.However, it is preferable that the main component of the ceramicparticles is the same as that of the dielectric layer 11. For example,BaTiO₃ having an average grain diameter of 50 nm or less may be evenlydispersed.

Then, the dielectric green sheet on which the internal electrode layerpattern is printed is stamped into a predetermined size, and apredetermined number (for example, 100 to 500) of stamped dielectricgreen sheets are stacked while the base material is peeled so that theinternal electrode layers 12 and the dielectric layers 11 are alternatedwith each other and the end edges of the internal electrode layers 12are alternately exposed to both end faces in the length direction of thedielectric layer 11 so as to be alternately led out to a pair of theexternal electrodes 20 a and 20 b of different polarizations. A coversheet to be the cover layer 13 is clamped to an upper face of thestacked dielectric green sheets, and another cover sheet to be the coverlayer 13 is clamped to a lower face of the stacked dielectric greensheets. The resulting stacked structure is stamped into a predeterminedsize (for example, 1.0 mm×0.5 mm).

After that, the binder is removed from the ceramic multilayer structurein N₂ atmosphere. After that, metal conductive paste for the externalelectrodes 20 a and 20 b is coated from the both end faces to the sidefaces of the ceramic multilayer structure and is dried. The metalconductive paste includes a metal filer, a co-material, a binder, asolvent and so on. The metal conductive paste is to be ground layers ofthe external electrodes 20 a and 20 b.

(Firing Process)

The binder is removed in N₂ atmosphere in a temperature range of 250degrees C. to 500 degrees C. After that, the resulting compact is firedfor ten minutes to 2 hours in a reductive atmosphere having an oxygenpartial pressure of 10⁻⁵ to 10⁻⁸ atm in a temperature range of 1100degrees C. to 1300 degrees C. Thus, each compound is sintered. In thismanner, the ceramic multilayer structure is obtained.

(Re-Oxidizing Process)

After that, a re-oxidizing process may be performed in N₂ gas atmospherein a temperature range of 600 degrees C. to 1000 degrees C. With there-oxidation process, the concentration of the oxygen defect is reduced.

(Forming Process of External Electrodes)

After that, with a plating process, a metal such as Cu, Ni, and Sn maybe coated on ground layers of the external electrodes 20 a and 20 b.With the processes, the multilayer ceramic capacitor 100 ismanufactured.

In the manufacturing method of the embodiment, the ceramic raw materialpowder satisfying the formula (1) and the formula (2) is used. It ispossible to, in the ceramic raw material powder, sufficiently enlargethe donor effect of the circumference region on whichreduction/oxidation caused by the firing atmosphere/thermal treatmentatmosphere has a large influence and to reduce the amount of the oxygendefect, when the multilayer ceramic capacitor 100 is manufactured byusing the ceramic raw material powder satisfying the relationships. Andit is possible to sufficiently enlarge the acceptor effect in the centerregion of the ceramic raw material powder. And the insulationcharacteristic is improved. It is therefore possible to achieve both thedonor effect and the acceptor effect. Accordingly, it is possible tomanufacture the multilayer ceramic capacitor 100 in which a balancebetween life and insulation characteristic is good and reliability isexcellent.

Although the embodiments of the present invention have been described indetail, it is to be understood that the various change, substitutions,and alterations could be made hereto without departing from the spiritand scope of the invention.

What is claimed is:
 1. Ceramic raw material powder comprising: a mainphase having a perovskite structure, wherein an element acting as adonor and an element acting as an acceptor are solid-solved in B sitesof the perovskite structure, wherein a relationship of (concentration ofthe element acting as a donor)×(valence of the element acting as adonor−4)<(concentration of the element acting as an acceptor)×(4−valenceof the element acting as an acceptor) is satisfied, in a center regionof each grain of the ceramic raw material powder, wherein a relationshipof (concentration of the element acting as a donor)×(valence of theelement acting as a donor−4)>(concentration of the element acting as anacceptor)×(4−valence of the element acting as an acceptor) is satisfied,in a circumference region of each grain of the ceramic raw materialpowder.
 2. The ceramic raw material powder as claimed in claim 1,wherein the main phase of the perovskite structure is BaTiO₃.
 3. Theceramic raw material powder as claimed in claim 1, wherein the elementacting as a donor is Mo, wherein the element acting as an acceptor isMn.
 4. A manufacturing method of a multilayer ceramic capacitorcomprising: forming green sheets including ceramic raw material powderof which a main phase has a perovskite structure; forming a multilayerstructure by alternately stacking each of the green sheets and each ofpaste for internal electrode; firing the multilayer structure, whereinan element acting as a donor and an element acting as an acceptor aresolid-solved in B sites of the perovskite structure, wherein arelationship of (concentration of the element acting as adonor)×(valence of the element acting as a donor−4)<(concentration ofthe element acting as an acceptor)×(4−valence of the element acting asan acceptor) is satisfied, in a center region of each grain of theceramic raw material powder, wherein a relationship of (concentration ofthe element acting as a donor)×(valence of the element acting as adonor−4)>(concentration of the element acting as an acceptor)×(4−valenceof the element acting as an acceptor) is satisfied, in a circumferenceregion of each grain of the ceramic raw material powder.
 5. The methodas claimed in claim 4, wherein the main phase of the perovskitestructure is BaTiO₃.
 6. The method as claimed in claim 4, wherein theelement acting as a donor is Mo, wherein the element acting as anacceptor is Mn.
 7. A multilayer ceramic capacitor comprising: aplurality of dielectric layers; and a plurality of internal electrodelayers, wherein each of the plurality of dielectric layers and each ofthe plurality of internal electrode layers are alternately stacked,wherein the plurality of dielectric layers are formed by firing theceramic raw material powder as claimed in claim 1.